1. Field of the Invention
The present invention relates generally to characterizing the performance of photolithography tools and more particularly to a method of determining the relative contribution of a photolithography tool's illumination optics and projection optics to across field line-width variation.
2. Related Art
Manufacturers of photolithography systems and in particular projection exposure systems are constantly striving to produce systems capable of creating semiconductor and other devices having the smallest possible line-widths. The line-width is a determining factor in the density which can be achieved in, for example, forming an integrated circuit (IC). As IC circuitry becomes more advanced, a greater number of circuit elements is typically required. Exposure systems must be capable of forming a greater density of circuit elements to avoid increasing the size of the IC. In addition to circuit density, smaller line-widths are also required for lower voltage operations and are desired to increase the speed at which the ICs operate.
Projection exposure system resolution is controlled by several factors, including the wavelength of light used to provide illumination for the exposure system, the numerical aperture of the projection lens or optical system (NA.sub.p1), and the coherence of the light from the light source. Those skilled in the art will appreciate that there is a trade-off between the maximum achievable resolution and the usable depth of focus in a lithographic system. In the case were an image needs to be reproduced on a given topography, the usable depth of focus can be a more significant factor to the user of the system than the achievable resolution. If the image cannot be reproduced on a given topography due to depth of focus limitations, the theoretical maximum achievable resolution will mean little to the user of the system. Thus, the usable depth of focus can often determine the overall performance of the lithographic system. Between the maximum achievable resolution and the depth of focus, there is generally an optimum trade-off point. The maximum resolution and depth of focus characteristics will combine to determine if a system is suitable for the image which is to be projected.
In addition to the projection lens system, projection exposure systems also include an illumination optical system having a numerical aperture NA.sub.i11. The illumination optics, which may include a lens, mirror, or other suitable optical element, receives the light from the projection system's light source and illuminates the mask or reticle to produce the image which is then focused by the projection lens or optical system onto an image plane where a substrate being subjected to the lithography process is placed. The ratio of the numerical aperture of the illumination optics to that of the projection lens or optical system (NA.sub.i11 /NA.sub.p1) is called the partial coherence factor (PCF), or sigma (.sigma.). It is known that the exact size of the projected image, depth of focus, and contrast across the image plan depend strongly on PCF and that PCF varies across the exposure field. Thus, the values of PCF or .sigma. found at various locations in the exposure field are not constant and can exhibit variation of as much as 10%.
The variation in PCF at the various locations in the exposure field, among other things, is known to contribute to variation in line widths or critical dimensions within the exposure field. This variation, commonly referred to as Across Chip Line-width Variation (ACLV) is also an important factor in determining a photolithographic tool's performance. Thus where typical design rules for fabrication of a semiconductor device require ACLV to be in the range of no greater than approximately .+-.10% of the nominal critical dimension for that device, such variations in excess of this amount can prevent a tool from achieving the needed results, thus interfering with production. It will be understood that for some applications ACLV variation of less than 10% can be desirable for microelectronic production.
In the related application entitled "METHOD AND APPARATUS FOR DETERMINING PARTIAL COHERENCE FACTOR IN LITHOGRAPHIC TOOLS", Ser. No. 08/818,375, (hereinafter '375) incorporated herein by reference, a method and apparatus which allows for rapid and reproducible measurements of variations in PCF or .sigma. across the image plane or exposure field in optical projection and lithography systems was presented. This method and apparatus was shown to be insensitive to other parameters or uncontrollable characteristics such as optical aberrations, defocus, exposure energy, photoresist properties or process conditions and advantageous with respect to previously known methods and systems.
Accordingly, to advantageously employ the method and apparatus of the above referenced application, there is a clear need for a method for determining the relative contribution of illumination and projection optics to across field line-width variations by making use of the PCF measurements obtained.